The liquid crystal display market has experienced rapid growth in many applications such as clocks, cellular phones, personal digital assistants (PDAs), notebook PCs, PC monitors, DVD players, and TVs. Liquid crystal display devices use liquid crystal switching to visualize the polarization state, and based on the display principle, they use polarizers. Particularly in TV applications and so on, higher brightness, higher contrast, and wider viewing angle are required, and polarizing films are also required to have higher transmittance, higher degree of polarization, and higher color reproducibility.
For example, iodine polarizers made of stretched polyvinyl alcohol (hereinafter, also simply referred to as “PVA”) to which iodine is adsorbed have high transmittance and high degree of polarization. Therefore, they are most popular polarizers widely used. A polarizing film commonly used includes a polarizer and transparent protective films bonded to both sides of the polarizer with a solution of a polyvinyl alcohol-based material in water, what is called an aqueous adhesive (Patent Documents 1 and 2 listed below).
A polarizing film can be produced using an aqueous adhesive such as a polyvinyl alcohol-based adhesive. In this case (what is called wet lamination), a drying step is necessary after a polarizer and a transparent protective film are bonded together. Such a method of manufacturing a polarizing film using an aqueous adhesive includes a drying step. Therefore, a transparent protective film with high water-vapor permeability, such as a triacetylcellulose film, is used in such a method. In addition, when an aqueous adhesive is used, the polarizer also needs to have relatively high water content so that the adhesive can have high tackiness to the polarizer. Otherwise, good tackiness cannot be achieved in the resulting polarizing film. Unfortunately, the polarizing film obtained in such a way has problems such as undesired optical properties (polarization properties) or degradation of durability against heating or humidification.
On the other hand, polarizing films can be easily influenced by humidity. When absorbing moisture from the air, transparent protective films can suffer from defects such as wavy curing or dimensional changes. In order to prevent a polarizing film from undergoing curling or dimensional changes in its storage environment, the water content of the polarizer may be reduced, or a transparent protective film with low water-vapor permeability may be used. However, if a polarizer and a transparent protective film with low water-vapor permeability are bonded together with an aqueous adhesive, drying efficiency will decrease because of the low water-vapor permeability of the transparent protective film, which makes it difficult to sufficiently remove water, as the solvent of the aqueous adhesive, by drying. As a result, the resulting polarizing film can have degraded polarization properties or insufficient durability, so that appearance defects can occur, which makes it impossible to obtain practically useful polarizing films.